More likely, this is being done to comply with SB 1119 (Adam's Law) [1,2] and similar legislation. These create extra steps that a service has to go through before releasing a new model that minors can interact with and exposes them to prosecution if a minor is harmed or harms someone else due to their interaction with a chatbot. It's safer for Anthropic to just exclude minors -- and they're unlikely to be paying customers anyway.
[1] https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml...
[2] https://sd18.senate.ca.gov/news/governor-newsom-signs-adams-...